The present invention relates generally to disc drives, and more particularly to determine the actuator imbalance torque as well as to locate the position of the imbalance torque.
The track following servo systems in disc drives are required to hold the read/write heads to very small off-track errors to support the increasing track density of today""s products. As errors can be induced due to many effects including disc and bearing runout, servo-track-writer induced irregularities, electronic noise, spindle and actuator resonances, and external shock and vibration excitations, modern disc drives must be designed and tested to perform satisfactorily to increasingly stringent specifications of shock and vibration. This is especially due to the needs of the portable computing market. Thus, there exists a need to be able to predict track following performance under external shock and vibration excitation. The ability to measure the actuator imbalance will be a good indication of the drive performance under shock and vibration condition.
In the existing setup to measure imbalance torque, the actuator is held to be free to rotate about its cartridge centre which is the pivot point and a load cell is located one inch from this point. The distance from the pivot point to the load cell multiplied by the force to move the actuator will give the imbalance torque. However, there are problems in using this method. Firstly, there is a need to remove the flex cable since it will contribute to the actuator an additional imbalance during measurement. Secondly, the location of the load cell may not be accurate enough to obtain a good measurement. Thirdly, the measurement could only be carried out at the component level.
Thus there remains a need for a more accurate and sophisticated method to determine the imbalance torque. It will be evident from the following description that the present invention offers this and other advantages.
The present invention relates to a more accurate method of determining the imbalance torque of an actuator in a disc drive at drive level and of locating the position of the imbalance torque using voice coil motor bias current values so as to diagnose the mechanical and servo performances of the disc drive.
According to one aspect of the invention, there is provided a method to sense the change in the voice coil motor (VCM) current to determine and to locate the position of the imbalance torque of an actuator in a disc drive. There are several steps performing different functions in the method. The first step of the method performs the function of obtaining a difference in the VCM current of the disc drive in the X orientation and in the Y orientation with the VCM current of the disc drive when the disc drive is placed flat. The second step performs the function of obtaining the square of each difference in VCM current. The third step performs the function of obtaining the imbalance current by adding the square of the VCM current and subsequent square root of the addition. The value of the imbalance torque is obtained by multiplying the imbalance current with the torque constant of the voice coil magnet. The fourth step performs the function of obtaining a division value of the VCM current in the X orientation or in the Y orientation with the VCM current of the disc drive when the disc drive is placed flat. The fifth step performs the function of obtaining the angle between the x-axis and the distance of the imbalance torque of the actuator from the pivot centre and measuring the mass of the actuator. The sixth step performs the function of obtaining the cosine or sine of the angle and multiple either angle with the mass of the actuator. The position of the imbalance torque of the actuator is located and determined either from dividing the divided value in X orientation in the fourth step with the cosine angle in the sixth step or dividing the divided value in Y orientation in the fourth step with the sine angle in the sixth step.
These and other features as well as advantages, which characterize the present invention, will be apparent upon reading of the following detailed description and review of the associated drawings.